Modulated radio-frequency (RF) signals are used in a wide variety of communications systems such as mobile phones, cellular base stations for mobile phones and satellite communications. In these applications, it is necessary to amplify the RF signal.
Conventional amplifiers have a generally linear relationship between input power and efficiency, and between input power and output power, during normal operation. As input power increases, efficiency and output power both increase. This means that high input power may be used to achieve high efficiency. Eventually, the amplifier is driven into saturation where the relationship between input power and output power is non-linear. In saturation, the output power does not increase as much.
It is undesired to operate an amplifier past saturation because driving an amplifier in the non-linear region also increases distortion of the signal. This means that when amplifying RF signals with high input power, conventional amplifiers cannot be used in the high-efficiency region, they have to be used in their linear region, where efficiency is much lower. To overcome this, Doherty topology amplifiers are used.
A Doherty amplifier has a main amplifier and a peak amplifier. The main amplifier is a class AB amplifier. The peak amplifier is a class C amplifier that begins to operate at higher input power levels when the main amplifier starts to saturate. With further increase of the input power, the peak amplifier modulates the load of the main amplifier in a way that maintains efficiency over a wide power dynamic range.
In use, an input signal is split and provided to the main amplifier and the peak amplifier. The outputs of the two amplifiers are combined to give an amplified output signal. At low input power, only the main amplifier is operational. At high input power, the peak amplifier is also operational. By using this arrangement, high efficiency can be provided.
RF signals in communications systems often have a high ratio of average power to peak power (sometimes 8 dB or higher). To ensure that the Doherty amplifier is operated in the linear region across the whole power range of the input, so that the amplifier is efficient and has reduced signal distortion, the Doherty amplifier is operated with the input power reduced from the maximum possible. This is known as operating the amplifier in back-off mode.
Doherty amplifiers with two peak amplifiers, also known as three-way Doherty amplifiers, have been used to improve the operating efficiency and power output in back-off mode. At low power, only the main amplifier is operational. At high power, both peak amplifiers are operational. At an intermediate power level, only one of the Doherty amplifiers is operational.
EP 2 403 135 to Alcatel Lucent proposes a four way Doherty power amplifier, which includes a main amplifier, and first, second and third peak amplifiers, the outputs of the amplifiers being coupled through an impedance network to an output node. The main amplifier is coupled to the output node through a first impedance inverting quarter wave transmission line. The first peak amplifier is coupled to the output node through second and third impedance inverting quarter wave transmission lines, in order to provide a resultant non-inverting impedance between the first peak amplifier and the output node, and the second peak amplifier is coupled to a node between the second and third transmission lines. The third peak amplifier is directly coupled to the output node. The output node is connected to an output load via an impedance transforming quarter wave transmission line.
In two, three, or four way Doherty amplifiers, the circuitry used to split the input signal and to combine the output signal such that the signals from the different amplifiers are phase and impedance matched causes a bottleneck that limits the frequencies over which the amplifier can be used. For example, in a cellular base station having allocated bands ranging from 0.7 GHz to 2.7 GHz, a Doherty amplifier may only have a relative bandwidth of the order of 5%, and thus each amplifier will only be able to operate over a single band. Therefore, to cover the whole bandwidth, a large number of different amplifiers have to be implemented which is costly in time and resources and inefficient in power consumption.